Tuned radio frequency receiver



Nov. 22', 1932. WWW B, R E I 1,888,514

TUNED RADIO FREQUENCY RECEIVER Filed March so. 1929 WALTER VAN B. ROBERTS 325 v is Suez m4;

Patented Nov. 22, 1932 UNITED STATES PATENT oFFIce WALTER VAN B. Ronrm'rs, or PRINCETON, NEW JERSEY, AssIGNoR ro R nIo CORPORATION or AMERICA, A conroR 'noNoF DELAWARE TUNED R nro FREQUENCY RECEIVER Applicatioufiled March 30, 1929. Serial No. 351,407.

This invention relates to tuned radio frequency receiving circults, and, more part1cularly, to a tuned radio frequency circuit in l which the coupling is varied coincidently 5 with the tuning of the circuit.

The characteristics of a stage of tuned radio frequency are first, amplification; second, selectivity; and third, the manner in which these quantities vary with frequency. The

amount of regeneration should also be considered, but this factor is really included in the amplificatiton and selectivity. The maximum non-regenerative amplification of any circuit is limited by the tube constants and the coil efliciencies. Almost any design of radio frequency amplifier can be made to operate ideally at all frequencies, if the coupling means is varied simultaneously with the tunmg.

Selectivity is limited by coil efliciency, and is further reducedby the coupling to the tube, there being a decided loss in selectivity if the coupling is adjusted for a' maximum amplification, but very little loss of selectivity occurring if the coupling is adjusted for a reasonable fraction, e. g. three-quarters ofmaximum amplification. Hence, in any type of amplifier, and at any frequency, the coupling determines what compromise there should exist between maximum amplification and maximum selectivity.

By having the coupling vary in a predetermined manner with frequency, it is possible Accordingly, it is one of themain objects of my present invention to provide means in a tuned radio frequency circuit, in addition to the usual tuning means, to secure a predetermined performance, for example, uniform to maintain maximum amplification at all voltage amplification, and contact band width selectivity. l

Another important object of the present invention is to provide means for securing ideal performance of a tuned radio frev quency receiving circuit. in which the coupling in said circuit is varied simultaneously in a predetermined manner with the usual tuning means.

The novel features which Ib'elievetobe characteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best I be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically two circuit organizations whereby my invention may be carried into effect.

In the drawing Figure 1 shows a circuit embodying my invention; and l V T Figure 2 shows a modified form of the invention. l i

The circuit indicated in Figure 1 comprises a receiving circuit with the usual antenna circuit, andincludes a stage T of radio frequency amplification and'a detector D for the sake of simplicity of showing. 'The -de I tector D is connected to any type of utilization means. The radio frequency stage T is coupled to a primary coil P inthe antenna circuit .bymeans of asecondary coil S, the latter forming with variable condenser G a tuned input circuit for the stage T.

.In the same manner the secondary coil S of the input circuit of detector D, forms with variable condenser G a tunedinput circuit for the detector. The condensers G G are mounted on; a common shaft in any .well known manner, shown on the drawing in dotted lines forthe sake of :simplicity, the common shaft being gearedto a shaft 6.. It is understood, of course, that the condenser shaft is operated by uni-control means (not shown), and that the uni-control tuning means is employed herein.

Thecoupling between coils P and S is maintained at the required value at each frequency by the rotation of a properly designedshield 1 mounted on an adjustment' shaft'3, the latter being geared, by means of a bevelled gear 8, to a bevelled gear 7 mounted .on the shaft 6. This results in the maintenance of a fixed relation between the adjustment shafted and the tuning means of the receiving OlI'Clllti-i, e f

, The shield 1 is preferably made of copper,

but may be made of any other metal suitable for the purpose, The shield 1' inay be positioned between'P and S, in which case, it not only varies the coupling, butalso reduces the apparent inductances of P and Sat high frequency. This is advantageous since, the first reduction lessens the tendency toward oscillation inlthetcase 0f inter-tube transformers, and the second reduction increases-the tuning range since; the reductionv occurs atthe high,

feed back (that is,;reve rsed with respect-to fseed back via plate-gridcapacity) to the coil The A on the shape of the rotor plate 5, the plate being mounted on the adjustment shaft 3 for rotation therewith, and the shape is "such as to give the desired amount of neutralization at each frequency. The shaft I 3 is ground, and operates simultaneously with the movement of the uni-control tuning means; The coupling between the neutralizing coil N and the coil Sis of any convenient-value V and is fixed, It is, however, possible to substitute'afixedcapacity for plates Land 5 and then-vary the coupling between N and S, preferably by y a properly shaped copper shield rotating on sh'aft 3 between the coils,

in similar-fashion to shield Exactly thesame mechamsm-may be used wherever tun'ed radio frequency amplification occurs,; -it-being understood that the shapes of -the shields 1, 2, and neutralizing condenser: plates 4,- 5 areto be changed 'accordinglto theparticularrequirements. For

examplef in ai superheterodyne it would be desirable to stress 1 maximum amplification rather than 'selectivity, because the selectivity is obtained in the intermediate frequency amplifier.- rOn theother hand, in a tuned radio frequency receiver'having many tubes, such as the present broadcast receivers,lpreferably designthe plates to give equahs'el'ectivity at all settings as the amplification is likely to be greater than necessary.

The operation of. the mechanism should be obvious from the above description and drawing, it being understood that'the shields 1, 2, and the condenser plates 4, 5 are sodesigned that for each frequency setting secured by rotation of the condenser common shaft, the adjustment shaft 3 simultaneously is brought into position to produce a coupling between the coils P, S, and the coils P fig, wherebythe de'sire damplification and selectivity are secured for'the said particular frequency,...it.. also being observed that the departure p ries in apredetermined fashion with the frequency due to the adjustment of the plates 4,5, coincidently with-the tuningmeans.

It is to be :noted that the various shields and plates maybe mounted on the same shaft that carries-therotors ofthe tuning means, I

in order to eliminate the gearing required to operatea' second shaft. e V.

ln Fig. 2, there is shown another embodi= ment of. the: invention. for maintaining uni-' form voltage amplification in stages-employ; ing tubes of the screen grid: alternating curfrom exact neutralization'varent type. As shown, the signal energy is fed I intoa tube 1,. the latter including a filament 3j indirectlyheated by a heater element i which is connected to thealternating current supply mains through a transformer 5,the midpoint of the transformer secondary beingi grounded, as is well known in the art.

-. The signal'energyisfed' into-the griafiia-f ment circuit through a resistance'R in series withthe' :filament, 1n a manner well known by those skilled in: the art. The plate current is fed through a chokei coil 6 from a source of direct current B,Lthe source B being also tapped at a. properpoint I to positively bias 'the-screenelements7 vof-tube's 1 and: 2,-it

being noted thatthe filament 3 of tube 2 is heated indirectly from the same alternating current source as tube 1. The output circuit of tuberl is coupled to the tuned input "circuit of tube-2 by a condenser 8 of small capacity. This capacity maybe, for example, approximately 20 micro-micrmfarads. .T he tuned input circuit of tube 2 ;includesthe inductance9 and variable capacity connected in parallel.

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'Someimagnetic couplingjbetween choke 6 and coil 9 maybe used, but iti snot essens tiaLTWiththis arrange'ment choke 6 preferably having a natural wave lengthgreater than thatof any of the desired signals, fairly un form voltag'e' amplification is obtained,

To make the amplificationperfectly uni- 'form,'a variable capacity 11 is connected across thechoke coil 6 and source B? in such a. manner that it "is in parallel with the distributed capacity 12 of the choke'coil 6, the capacity 12being shown in dotted'lines. It is to be noted that the condenser 11 is mounted so that its rotor is uni-controlled along with the shaft 13 (shown as a dotted line) upon which shaft the rotor of tuning condenser 10 is mounted.

The plates of condenser 11 are designed so as to give the greatest capacity Where the amplification would otherwise be greatest. It has been ascertained by experiment that at any frequency the amplification may be reduced to the extent required to maintain the desired uniformity, by having the condenser 11 of a very small capacity of the order of 10 micro-micro-farads or less. Thus by this arrangement, no mechanical difliculty is encountered in providing a condenser that will have the proper capacity at such frequency.

While I have indicated and described only two systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications in the circuit arrangements, as well as in the apparatus employed, may be made Without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

1. In a receiving circuit a space discharge device having an anode, a cathode, a control electrode and a shielding electrode, a source of current and a choke coil in series connected between the anode and the cathode, a second space discharge device having a tuned input circuit and an output circuit, said tuned input circuit comprising a variable capacity, a condenser for connecting the anode of said first mentioned space discharge device to one side of said variable condenser, a second variable condenser shunted across said choke coil and said current source, one side of each ofsaid variable condensers and one side of said current source being grounded.

2. In a receiving circuit, a space discharge device having an anode, a cathode, and a control electrode, a circuit comprising a source of current and a choke coil in series connected between the anode and cathode of said space discharge device, said circuit having a natu ral wave length greater than that of any signals desired to be received, a second space discharge device having a tuned input circuit, said tuned input circuit comprising a variable condenser, a capacity for coupling the anode .of said first mentioned space discharge device to the control electrode of said second named device, a second variable condenser shunted across said choke coil, and means for connecting one side to each of said variable condensers and one terminal of said current source to ground.

3. In a receiving circuit, a space discharge device having an anode, cathode, and a control electrode, a circuit comprising a source of current and a choke coil in series connected between the anode and cathode of said space discharge device, said circuit having a natural wave length greater than that of any signals desired to be received, a second space discharge device' having a tuned input circuit, said tuned input circuit comprising .a variable condenser, a coupling condenser for connecting the anode of said first mentioned space discharge device to one side of said variable condenser, a second variable condenser shunted across said choke coil, one side of each of said variable condensers and one terminal of said current source being grounded. I p

4. In a receiving circuit, an electron discharge device having a cathode, an anode and a control electrode, a coil having inherent distributed capacity, connected between said anode and said cathode forming a circuit having a natural wave length greater than that of any signals desired to be received, a variable condenser connected in parallel with the inherent capacity of the coil','said variable condenserhaving a capacity of the order of magnitude of 10 micro-micro-farads, a second space discharge device having a tuned input circuit comprising an inductance coil and a variable tuning condenser, a coupling condenser connected between one end of said first named coil and one end of said tuned input circuit and means for simultaneously varying said two variable condensers. p

5. In a receiving circuit an electron discharge device having a cathode, an anode, a control electrode and an auxiliary electrode, a coil and a source of current in series connected between said anode and said cathode, said coil having suflicient inherent distributed capacity to form a circuit having a natural wave length greater than that of any signals desired to be received, a connection between said auxiliary electrode and a point of said source of intermediate potential, a variable condenser connected across said coil and current source, a second space discharge device having a cathode, an anode, a control elec trode and an auxiliary electrode, a tuned circuit comprising an inductance and a variable condenser connected across the control electrode and the cathode of said second device,

means for connecting said two cathodes to ground, means for simultaneously varying both said variable condensers and a coupling means comprising a condenser connected between the anode of said first named device and the control electrode of said second named device. r

, WALTER VAN B. ROBERTS. 

